Poly(acrylic acid) homopolymer and copolymer rheology modifiers are often provided in solid, pulverulent form. They are used in many technical fields (e.g., coatings, paper production, textile industry, personal care, household care and in the pharmaceutical industry). These materials are conventionally prepared by polymerizing acrylic acid and an optional crosslinker (with or without other comonomers) with a free radical initiator in an organic medium contained in a reaction vessel equipped with stirring means. The organic medium is a solvent for the monomers but a substantially non-solvent for the resulting polymers. Shortly after the initiation of the polymerization reaction and during the course of the polymerization, nascent polymer particles begin to precipitate from the solution, flocculate and form aggregates. The precipitated polymer forms a slurry in the solvent which often becomes extremely viscous, resulting in ineffective mixing, restricting of monomer to free radicals, poor heat transfer, and polymer fouling on the reactor surfaces ultimately limiting the total solids level (i.e., polymer yield) that can be obtained in industrial scale production equipment. This condition can occur in many solvents at relatively low solids levels (e.g., 8 to 10%). An additional problem is that the solvent becomes entrained in the polymer during the aggregation of the particles making it difficult to obtain the desired dried polymer product, requiring excessive cycle time and energy consumption to remove the solvent in an environmentally acceptable way. An improved solvent polymerization system is desired.
Various processes have been proposed for producing particles of crosslinked acrylic polymers stably dispersed in an organic solvent. According to conventional processes, dispersed polymer particles are produced in the presence of a dispersion stabilizer polymer composed of a first segment which is solvated by the organic solvent employed in the reaction medium and a second segment which is substantially non-soluble in the organic solvent employed but serves as a portion that becomes “associated” to the dispersed polymer particles. The solvated segment of the dispersion stabilizer polymer extends sufficiently away from the polymer particle forming a steric repulsive barrier to prevent the aggregation or cohesion of individual polymer particles thereby stabilizing the polymer in dispersion. The associative force between the stabilizer and the disperse polymer may, for example, be a mass-dependent force generated between components of the stabilizer which are not solvated by the organic solvent and the disperse polymer (e.g., intermolecular entanglement). It may be a force generated by strong specific interaction between polar groups in the stabilizer and complementary polar groups in the disperse polymer (e.g., hydrogen bonding, Van der Walls forces). It may be the attraction between oppositely charged ionic groups in the stabilizer and in the dispersed polymer (e.g., ionic attraction). Alternatively, it may be a covalent bond between the non-solvated segment of the stabilizer and the disperse polymer (e.g., grafting, reaction via olefinic unsaturation in the non-solvated segment of the stabilizer reacting with the growing disperse polymer chain, condensation reaction, etc.).
U.S. Pat. No. 4,375,533 to Park et al. discloses a process for overcoming some of the foregoing problems. The Park et al. process for the polymerization of acrylic acid and optional comonomers in an organic media is characterized by the use of nonionic surface active agents having HLB values between 1 and about 10, including surface active agents containing poly(ethylene oxide) ether moieties.
Similarly, U.S. Pat. No. 4,419,502 to Sehm discloses a process for the polymerization of acrylic acid and optional comonomers in the presence of a nonionic surface active agent selected from polyoxyethylene alkyl ethers and polyoxyethylene sorbitol esters and having an HLB value greater than 12.
U.S. Pat. No. 4,420,596 to Lochhead et al. discloses a process for polymerizing carboxylic acids in mineral spirits. This process also employs nonionic surface active agents having HLB values less than 10. Among the disclosed surface active agents are (1) sorbitan ester, (2) glycerol or alkylene glycerol ester, and (3) long chained alcohols.
U.S. Pat. No. 4,526,937 to Hsu teaches the polymerization of acrylic acid in an organic solvent with a free radical catalyst. This process incorporates nonionic block copolymers of propylene oxide and ethylene oxide to minimize undesirable flocculation and agglomeration during the reaction process.
U.S. Pat. No. 4,692,502 to Uebele et al. discloses a process for the polymerization of unsaturated carboxyl containing monomers such as acrylic acid and optional comonomer in the presence of a free radical forming catalyst and at least one ionic surface active agent selected from anionic, cationic and amphoteric agents.
U.S. Pat. No. 5,288,814 to Long, I I et al. describes interpolymers of acrylic acid and optional comonomers which are polymerized in the presence of a steric stabilizer surfactant having at least one hydrophilic moiety and at least one hydrophobic moiety arranged in a linear block copolymer configuration or a random comb copolymer configuration. Both steric stabilizer polymer configurations contain hydrophilic moieties comprising polyoxyethylene ether groups. The solid acrylic based polymers obtained from the disclosed polymerization procedure are characterized by their ease of handling and the ability to be easily dispersed in aqueous media.
U.S. Patent Application Publication No. 2011/0150796 to Kim et al. discloses a process for preparing a crosslinked copolymer via free radical precipitation polymerization of a monomer composition containing acrylic acid and other copolymerizable monomers. The precipitation polymerization reaction is performed in the presence of at least two auxiliary components selected from glycerol monostearate and at least one compound having an HLB value ranging from 4 to 10, chosen from water insoluble waxes, nonionic emulsifiers and combinations thereof.
While the prior art has attempted to solve several of the inherent problems associated with polymerizing crosslinked acrylic based polymers in organic media, there is still a need for an efficient process for producing such polymers in high yield and without the excessive reaction media viscosity increases and concomitant reactor fouling. The polymer precipitate must be easily recoverable in powdered solid form in subsequent isolation steps and mucilages of these polymers must possess good clarity, structure (e.g., texture), and dissolution properties.